Viruses for therapeutic and diagnostic methods often are pathogenic and must be attenuated to increase their safety for administration. Attenuation can be effected by repeated passage through cell lines and/or through animals to screen for strains that have reduced pathogenicity. Other methods for attenuation of a virus involve production of recombinant viruses that have a modification in one or more viral genes that results in loss or reduced expression of a viral gene or inactivation of a viral protein. Once attenuated viruses are generated, methods for increasing the attenuation of the virus often involve selecting or identifying additional genes for mutation, combining mutations and/or insertion of heterologous genes for expression of proteins that alter the in vivo pathogenicity of the virus (see e.g., U.S. Pat. No. 6,265,189 and U.S. Patent Publication No. 2006-0099224). The effects of combinations of modifications, however, are difficult to predict and require extensive testing to determine what combinations of modifications yields a desired level of attenuation. Further complicating the process is the fact that mutations often decrease or abolish viral functions that are required for viral replication or life cycle progression. Essential viral functions often are provided in trans in order to produce the mature virions for infection (see e.g., U.S. Pat. Nos. 5,750,396, 6,261,551, 6,924,128, 6,974,695). Thus, packaging cell lines that express the essential viral proteins are required for viral propagation. Such cell lines, however, can be challenging to generate due to the toxicity of the viral proteins that are expressed.
Mutation of non-essential genes is a method of attenuation that preserves the ability of the virus to propagate without the need of a packaging cell lines. In viruses such as vaccinia virus, mutations in non-essential genes, such as the thymidine kinase (TK) gene or hemagglutinin (HA) gene have been employed to attenuate the virus (e.g., Buller et al. (1985) Nature 317, 813-815, Shida et al. (1988) J. Virol. 62(12):4474-80, Taylor et al. (1991) J. Gen. Virol. 72 (Pt 1):125-30, U.S. Pat. Nos. 5,364,773, 6,265,189, 7,045,313). The inactivation of these genes decreases the overall pathogenicity of the virus without eliminating the ability of the viruses to replicate in certain cell types. Further modulation of the attenuation of the virus similarly is difficult, since it can require identification of additional non-essential genes for modification, followed by testing of combinations of mutations in order to select a recombinant virus with a desired level of attenuation.
In view of the efforts to generate attenuated viruses for therapy, including the methods mentioned above, there still exists a need for attenuated viruses. Accordingly, it is among the objects herein, to provide attenuated viruses that can be employed for diagnostic and/or therapeutic methods.